A conventional X-ray tube of the rotating anode type for medical applications, comprises a housing with a vacuum envelope disposed therein. An anode target is mounted on a shaft assembly for rotation within the vacuum envelope. A cathode assembly is disposed within the vacuum envelope in the vicinity of the anode target. High voltage source is connected to the anode target and the cathode assembly. A cloud of electrons emitted from the cathode are accelerated to high energy and hit the anode target at a focal spot. The anode target emits X-rays in response to the incident electrons. When electrons strike the anode target only a small fraction of their energy is converted to X-rays, while the major portion of the energy is released as heat, thereby elevating the anode target temperature in operation. In order to distribute the thermal load the shaft assembly with the anode target is rotated at approximately 3,000 to 10,000 rpm. The shaft is coupled to the vacuum envelope via bearings. High rotation speeds and accelerations of the anode structure generate vibration of this structure which is transmitted to the vacuum envelope. Since the envelope has a relatively large size it is the primary source of further vibration transmission to the housing surrounding the envelope. The oil which is filled between the envelope and housing for heat dissipation and dielectric purposes transmits a significant portion of the vibration to the housing and it is radiated as acoustic noise. These factors limit the service life of the X-ray tube and cause disturbances to the personnel in the vicinity.
A number of technical decisions have been proposed in the past to achieve X-ray noise reduction. These include lowering the rotation speed of the target, adding a mass on an anode shank mounting area, as well as other techniques.
A prior art design for reducing X-ray tube noise without shortening target life expectancy by lowering the target speed rotation area described, for example, in the U.S. Pat. No. 4,935,948 "X-ray Tube Noise Reduction by Mounting a Ring Mass", and the U.S. Pat. No. 5,265,147. In the U.S. Pat. No. '948, a ring mass is attached on or near the bearing shroud which physically connects the rotor bearing to the vacuum tube Such a ring mass significantly increases the overall weight of the X-ray tube, and, being located near a high voltage region within the tube, creates electrical instability. In the U.S. Pat. No. '147 X-ray tube noise is reduced by sealing the stator mass to the neck portion of the glass vacuum envelope or clamping it with a mechanical clamping device.
An alternative approach to noise reduction in X-ray tubes is described in U.S. Pat. No. 5,253,284, "X-ray Tube Noise Reduction Using Non-Glass Inserts". Although satisfactory in certain respects, such X-ray tubes shall suffer from disadvantages. Thus, the rigid ring, which is used as an insert, requires extensive redesign of an X-ray tube, and does not isolate the vacuum envelope from the source of vibration.
Yet another conventional approach to noise reduction relies on disposing vibration damping means outside X-ray tube. An application of this approach is disclosed in the U.S. Pat. No. 4,433,432 "X-ray Tube Apparatus", where the end portion of the rotary anode X-ray tube is resiliently supported and is equipped with vibration damping means. This vibration damping means is engaged with the bearings of an anode target. Such a design does not disconnect the glass vacuum envelope from the rotating anode vibration source.